This invention is directed to a flexible superabsorbent binder polymer composition, and a method of making the flexible superabsorbent binder polymer composition and absorbent binder or coating composition including the superabsorbent polymer composition.
Adhesives, or binders, are a necessary element of many absorbent products. While adhesives beneficially hold products together, adhesives may also have a tendency to interfere with the absorbency of fluids in absorbent products. Adhesives are typically hydrophobic and therefore are not conducive to absorbency or liquid transfer functions. Furthermore, most adhesives are non-absorbent and thus serve no liquid retention function.
Hydrophilic adhesives are known, such as adhesives formulated from water-soluble polymers such as poly(vinyl alcohol), poly(vinyl methyl ether), poly(vinyl pyrrolidone), poly(ethylene oxide), or cellulose derivatives such as hydroxypropyl cellulose. Dextrans, starches and vegetable gums have been used to provide hydrophilic adhesives. These materials provide adhesion under dry conditions. However, upon exposure to aqueous fluids, these materials lose bonding capability because they are substantially soluble in aqueous fluids.
A known approach for making hydrophilic adhesives more functional upon exposure to aqueous fluid is to crosslink the water-soluble polymers. As a result of crosslinking, the material becomes swellable, and no longer soluble, in aqueous fluid. However, since crosslinked polymers are thermosets, they are difficult to apply to substrates or to establish intimate contact with surfaces because the crosslinked polymers are solid materials and have little or no ability to flow. Usually crosslinked polymers are hard, rigid and brittle. These polymers do not have flexibility that many absorbent products need for end uses of the polymers.
What is therefore needed is a flexible superabsorbent binder polymer composition, which may be referred to as flexible absorbent binder or FAB herein, for use in a hydrophilic flexible binder or coating that has latent crosslinking capability. Such a superabsorbent polymer composition could be easily applied, like a water-soluble polymer, since the hydrophilic polymer solution would be capable of flow prior to crosslinking. Latent crosslinking capability would also provide a simple means of crosslinking the polymer after the polymer has established intimate contact with substrates or has formed a desired final shape or form.
Recent development efforts have provided coating materials for a variety of uses. For example, U.S. Pat. No. 6,054,523, to Braun et al., describes materials that are formed from organopolysiloxanes containing groups that are capable of condensation, a condensation catalyst, an organopolysiloxane resin, a compound containing a basic nitrogen, and polyvinyl alcohol. The materials are reported to be suitable for use as hydrophobic coatings and for paints and sealing compositions.
Soerens et al., in U.S. Pat. No. 6,737,491, describes an absorbent binder composition including a monoethylenically unsaturated polymer and an acrylate or methacrylate ester that contains an alkoxysilane functionality that is particularly suitable for use in manufacturing absorbent articles. Also described in Soerens et al. is a method of making the absorbent binder composition that includes the steps of preparing a monomer solution, adding the monomer solution to an initiator system, and activating a polymerization initiator within the initiator system reported an alcohol-based, water-soluble binder composition. One of the issues in preparing water-soluble polymers is the amount of the residual monoethylenically unsaturated monomer content remaining in the polymer. For applications in personal hygiene it is required the amount of residual monoethylenically unsaturated monomer content of the superabsorbent polymer composition be less than about 1000 ppm. In addition, it is desirous for the flexible superabsorbent binder composition to have a solids content of at least 24%. Also it is desirous to have a uniform and reproducible viscosity at the solids level of greater than 24%.
In addition to the foregoing, another aspect of the existing flexible absorbent binder composition is that a relatively dilute polymer solution must be used so that the solution does not begin to crosslink, wherein crosslinking is known as gelling. Since the solution is generally used by applying the solution to a substrate and then drying the solution to form the crosslinked, absorbent coating, the cost of drying a dilute solution may be more costly than desirous.
In the case of using alkoxysilane as a crosslinking agent, gelling can be explained, without this invention being limited by the following theory, as a function of the statistical probability that the alkoxysilane functional units on two polymer chains “find” each other in solution to allow condensation and formation of a crosslink. One approach is to keep this probability low by using a polymer concentration of about 20% by weight. Another approach is to reduce the probability of gellation by reducing the amount of alkoxysilane functionality incorporated into the polymer. For example, a simple approximation suggests that incorporating half as much alkoxysilane into the polymer should allow the polymer concentration to be doubled. Thus a 40% polymer solution with half the alkoxysilane incorporation should have the same stability as the current 20% solution of flexible superabsorbent binder polymer composition. However, the 40% solution requires much less energy and cost to dry the solution into an absorbent coating. Reducing the alkoxysilane incorporation, however, reduces the overall crosslink density of the absorbent coating. A “sloppy” absorbent that is too low in gel strength for functional absorbency is likely to result. As set forth in the present invention, this sloppiness can be overcome by adding a new source of crosslinking functionality just before the concentrated binding/coating solution is applied to a substrate.
What is therefore needed is a hydrophilic polymer that has latent crosslinking capability and which can be produced at an attractive cost. Such polymers could be easily applied, like a water-soluble polymer, since the hydrophilic polymer would be capable of flow prior to crosslinking. Latent crosslinking capability would also provide a simple means of crosslinking the polymer after the polymer has established intimate contact with substrates or has formed a desired final shape or form. There is also a need or desire for such a polymer which has a suitable level of flexibility.
In addition, there is thus a need within the field of absorbent products for flexible absorbent binders, adhesives, or coatings from a polymer solution wherein the superabsorbent polymer composition has a residual monoethylenically unsaturated monomer of less than about 1000 ppm, and less than about 500 ppm. In addition, it is an objective of the present invention to increase the solids content of the polymer solution to at least about 24%, and at least about 30%. Another objective of the present invention is to have a viscosity after 16 hours of the superabsorbent polymer composition of less than about 10,000 cps, and in the range of from about 1,000 to about 5000 cps.
Furthermore, there is a need within the field of absorbent products for such flexible absorbent binders, adhesives, or coatings that can be prepared by post-application, moisture-induced crosslinking of superabsorbent polymers.